Various devices are known for reducing, minimizing or eliminating human exposure to high levels of electric fields (E-fields) in frequency ranges of 0 to 400,000 Hz, i.e., to E-fields of ELF (extremely low frequency, in the range of 0-3 KHz) and VLF (very low frequency, in the range of 3 KHz-400 KHz). Exposure to these fields may induce undesirable biological effects in humans. It is known that E-field shields require a connection to electric ground. Thus, known shields all include and require a ground connection.
Known shielding devices include faraday cages, coaxial shields, shielded multiconductor cables and E-field shielding plates. A common characteristic of known shielding devices is the use of a conductive shield, surrounding or obstructing passage of electric field lines. In the prior art, however, shielding devices are all provided with a ground connection.
Because of safety or regulatory provisions, some devices, such as electrically heated bed covering for example, are precluded from having a ground connection. Accordingly, although reduction of electric field emissions by heated bed covering is a desirable goal, it has been impossible to meet such a goal in view of regulatory prohibition of a ground connection to such bed covering.
In general, there exist numerous electrical devices or appliances which, whether because of double insulation, the use of a two blade polarized connector, or for other reasons, are not grounded. While electric and magnetic fields are produced by presence of electrical voltage and current, respectively, in devices such as electrically heated bed coverings, video display terminals and double insulated appliances, such devices can not use a three blade plug or other connection to safety ground of the electrical power distribution system. Accordingly, such devices have been left without any E-field shielding by the prior art.
One reason that E- field shielding has not been provided in certain electrically powered devices is the very inability to use a ground connection, and the resulting attendant risk of personal injury or damage to persons or property coming into physical contact with ungrounded shields, as well as the conventional wisdom that electrical conductors must be grounded to provide effective shielding.
For example, a faraday cage is known as an effective shield for reducing ELF and VLF E-field emissions from an AC electrically-powered device. Such a shield, shown in FIG. 6, is formed of a thin walled, conductive enclosure 10 which completely surrounds the E-field emitting object (or appliance) 12. If more than one piece of conductive material is used to fabricate a single layered faraday cage, all the pieces, or sheets, of conductive material are electrically connected along common adjacent borders. As seen in FIG. 6, the faraday cage requires a ground connection 14.
The wires or conductors 15 which connect the appliance in the faraday cage to the AC power distribution system must also be shielded using individual coaxial shields or a shielded multiconductor cable, separately described below.
Thus, while a faraday cage can be used to reduce E-field emissions from AC powered devices, such a cage is only used in the prior art in conjunction with a ground connector 14, connecting the cage to safety ground of the power distribution system.
Another known form of shield, which is a special form of the faraday cage, is a coaxial shield used to shield a single electrical conductor or wire. Such a shield is shown in FIG. 7, wherein an inner conductor 16 to be shielded is surrounded by a non-conducting material 18, such as air, a vacuum, a special dielectric material, or insulating material. An outer conductor 20, or shield, is then wrapped around and surrounds the non conductor 18. The shield 20 is typically composed of a wire and foil combination, or braided wire that forms a thin, continuous layer completely surrounding the inner conductor 16, with dielectric 18 therebetween. An insulating outer jacket 21 surrounds conductor 20. The coaxial shielding technology of FIG. 7 can be used in an AC electrical power distribution system to reduce E-field emissions.
In such an application, inner conductor 16 is the wire carrying current to the electrical load, and shield 20 acts as the second conductor in the electrical circuit, carrying the current back from the load to the source.
However, in the prior art, when an arrangement such as shown in FIG. 7 is used as an E-field shield in an AC powered electrical system, the outer conductor 20 is connected to safety ground of the distribution system.
A shielded multiconductor cable is a special form of the faraday cage or the coaxial conductor. Such a shielded multiconductor cable is illustrated at FIG. 8, and is used to reduce ELF and VLF E-field emissions from multiple wires or conductors routed to the same destination. In such a shielding approach, an outer conductor 20 is used which is identical to that used in the coaxial shield of FIG. 7. However, rather than surrounding a single conductor, the shield surrounds a plurality of inner conductors 22. The outer conductor has a circular cross section and is typically composed of wire and foil or braided wire in a thin, continuous layer which surrounds completely all of the inner conductors 22, as seen in FIG. 8. An outer conductor 20 for a multiconductor cable may have an irregularly shaped cross section.
Shielding of multiconductor cables as shown in FIG. 8 is used in AC electrical power distribution systems to reduce E-field emissions. In such an application, the plural inner conductors 22 carry currents to various electrical loads. Additional inner conductors (not shown) can also be used to carry the current back from each load to the AC power source.
However, when an arrangement such as shown in FIG. 8 is used as an E-field shield in the prior art, outer conductor 20 is connected to safety ground of the AC electrical distribution system, similarly to the required connections for FIGS. 6 and 7.
Yet another known shield, shown in FIG. 9A, is used to provide a partial reduction of ELF and VLF E-field emissions from certain AC electrically powered devices. Specifically, an E-field shielding plate 24 is used in conjunction with an electrically powered appliance such as a video display terminal (VDT) 26 used with a computer. Such shielding plates are frequently sold as an accessory for VDTs.
The shielding plate 24 typically uses a thin, optically-transparent and electrically-conductive film, or a thin wire mesh embedded in an optically transparent glare reducing plate. The plate 24 is placed in front of a viewing area of a VDT 26, as shown in FIG. 9A.
In order to make the shield plate 24 effective at the AC power line frequency, the prior art requires, and uses, a separate wire 28 (with or without a current limiting resistor) to connect shield plate 24 to an electrical grounding point, i.e., safety ground of the electrical power distribution system.
Thus, each of the known E-field shielding devices requires a connection to safety ground of the electrical power distribution system.
The above described known E-field shields thus cannot be incorporated in AC appliances that are double-insulated and/or use a two-wire electrical power cable and a two-blade plug. As hereinabove noted, the known shields require direct connection of a conductor from the shield to the safety ground of the AC power distribution system. In order to provide such connection, a known third (ground) connector of known three prong AC electrical plugs and outlets is used. Thus, AC powered devices using the known E-field shields can only be used at locations wherein an electrical power outlet has a three conductor receptacle, such as accommodates a three blade plug, with direct connection to safety ground. Alternatively, a trained electrician or technician must connect the shield to safety ground, using special adapters or wiring.
As above noted, however, there are some appliances which do not or cannot provide such a connection to safety ground. For example, UL ANSI 964 provides a standard for electrically heated bed coverings. The standard states that any electrical power wiring and associated attachment plug may contain two or more circuit conductors, but explicitly excludes a grounding conductor. Similar wiring is required for an interconnecting cord between a heating control unit and the bed covering or for other interconnections used in conjunction with the heated bed covering. Ground conductor prohibitions are required, in several instances, to prevent acute injuries, such as electrical shocks or burns due to heat generated by electrical short circuits, to persons who may physically contact a portion of the shielded appliance.
Thus, there is regulatory prohibition of connection of any grounding conductor to electrically heated bed coverings. Accordingly, E-field shielding has not been available for such an appliance, for other appliances similarly limited, or for double insulated appliances which are similarly prohibited from direct connection to safety ground via a separate grounding wire.
In order to define the various aspects, advantages and features of the invention, the following explanation is provided for several of the concepts associated with the invention, as they relate to grounding of standard AC electrical power distribution systems.
Referring to FIG. 10, there is illustrated an AC electrical outlet socket 30, having three connection receptacles. It is known that every AC electrically powered device has a first connection 31 to a source of electrical power. In a standard, properly wired, three receptacle electrical outlet 30, the ac source is connected to the shorter receptacle that mates with the narrower blade of the device's mating plug 51. It is also known to those skilled in the art that every AC electrically powered device must also have a direct connection to the neutral, or return conductor of the AC power distribution system. This connection consists of a wire or other conductor that carries all of the AC current flowing from the electrically powered device back to the AC distribution system. No device or appliance is able to operate properly and safely without being connected to the neutral, or return point of the ac power distribution system.
In FIG. 10, the neutral conductor of the AC power distribution system is connected to the longer receptacle 32 that can only mate with the wider-tipped flat blade 49 of the device mating plug. An optional safety ground, or grounding conductor 34, is typically incorporated in an AC power distribution system. The safety ground is a third conductor, routed from a single zero-voltage grounding point in the electrical distribution system to each individual electrical outlet in a house or workplace.
It is known that safety grounding conductors never intentionally carry current to or from an electrically powered device. Moreover, an appliance or electrically operated device need not be connected to a safety ground wire, or to a safety grounding conductor 34 of an electrical outlet, in order to operate. Electrical appliances function in a normal manner without such a connection.
Safety ground connectors only carry current if an unsafe, abnormal ground fault exists in the electrical distribution system, or in an electrical device connected to the system. Thus, ground fault interrupters are known devices which sense current flow in the ground connector to interrupt current flow in the ac terminals, in order to avoid a hazardous condition.
Based on the foregoing, the following description identifies various difficulties of the prior art and the solutions therefor provided by the present invention.
Electrically-powered devices operating without an E-field shield always emit electric fields of varying magnitudes when energized with AC power. When a device is connected to a 100 volt AC power source, for example, such fields may have intensities exceeding 100 volts per meter, when measured-under non-perturbed or "free field" conditions. Inasmuch as such E-fields induce currents in the bodies of nearby persons, it is desirable to avoid exposures of humans to high levels of ELF and VLF E-fields since such exposures can, under certain conditions, induce biological changes or effects.
Moreover, unshielded devices with non-polarized plugs or connectors, wherein either conductor of the appliance may be connected to the AC source connector 30 and either may be connected to neutral connector 32, have a 50% probability of emitting strong electric fields even when in an "OFF" condition and not operating. In a similar manner, unshielded devices having polarized connectors, but having non-polarized intermediate connectors, also can generate E-field emissions when the devices are in an "OFF" condition if the power switch, which is normally single-pole, is in series with the wire connected to the neutral connection 32 of an outlet by the intermediate connectors.
As will be appreciated by persons of ordinary skill in the electrical arts, both a non-polarized connector for connection to an AC outlet and a non-polarized intermediate connector may place the control switch for a device on the neutral line. Thus, even when the switch is open, the AC source connection provided at receptacle 30 is often connected to a conductor of the device, resulting in E-field generation even when the device is "OFF".
Unfortunately, such non-polarized plugs and intermediate connectors, which can be connected to a mating electrical receptacle in either of two different physical orientations, are used on a large number of electrical devices and appliances.
There is thus a need in the prior art for E-field shielding devices for ungrounded appliances.
There is a further need in the prior art for devices to prevent generation of E-fields by appliances having non-polarized plugs or intermediate connectors, and particularly to prevent generation of such E-fields when the appliance is in an "OFF" condition.
There is a more particular need for E-field shields for electrically heated bed coverings and other appliances precluded from having a ground connection.